1. Field of the Invention
The present invention relates to a magneto-optical disc recording method for use in recording audio and video data on a magneto-optical disc (MO disc). The present invention also relates to a magneto-optical disc recording and reproducing apparatus therefor.
2. Description of the Prior Art
In conventional optical discs that are utilized in a CAV (constant angular velocity) mode, each track has at a predetermined position periodically provided with a servo byte interval in which there are formed a clock pit for generating a reference clock and a wobbled pit for effecting tracking. A reference clock (channel clock) is generated in association with the clock pit and information is digitally recorded by means of a pit whose length corresponds to integral times of the period of the reference clock. Although a system such as a CD (compact disc) utilized in a CLV (constant linear velocity) mode has no clock pit, length and bit interval of a recorded pit are selected to become as long as an integral number of times (lengths of nine kinds in a range of from 0.9 .mu.m to 3.3 .mu.m in the case of CD) of the period (0.3 .mu.m) of the reference clock (channel clock), whereby the clock is reproduced and recorded information is cut in bit units.
In video discs that are also optical discs, a video signal is recorded and/or reproduced so that the difference in length of the pits are far smaller than that of the CD. This phenomenon will be described below in association with an example of a signal that is recorded on the video disc at a radial position of 55 mm according to the CAV mode. In video discs, a brightest portion of a video signal is recorded as a signal of 9.3 MHz and a darkest portion thereof is recorded as a signal of 7.6 MHz, These frequencies of 9.3 MHz and 7.6 MHz correspond to lengths of 1.075 .mu.m and 1.316 .mu.m on the video disc having a radius of 55 mm, respectively. It is a well-known fact that, if the disc thus recorded is reproduced, then a high quality picture can be reproduced. Assuming that the change of brightness can be expressed with 128 tones in the above reproduced picture, then it is to be appreciated that this means that the period of pits recorded on the video disc corresponding to the 128 will require sensitivity to very small variations in pit length. That is, the changes of pit length and pit interval of (1.316 .mu.m-1.075 .mu.m).div.128 =0.002 .mu.m are reflected in the video signal.
The reason that the minimum unit of the change of the pit length is inevitably selected to be as large as 0.3 .mu.m in the (CDs although very small changes in pit length can be recorded) is based on the assumption that the signal will not be recorded and/or reproduced by an optimum recording and/or reproducing method.
The assignee of the present application has previously proposed a method in which the position of a front end (forward) or rear end (backward) edge of the information pit is stepwise shifted from a predetermined reference position in response to information to be recorded (see Japanese patent application No. 3-167585). According to this previously-proposed recording and reproducing method, the changes of pit length and pit edge position can be detected with very high accuracy so that it becomes possible to record information utilizing very small changes in pit length unlike the prior art. Consequently, higher density recording can be realized.
FIGS. 1A to 1C of the accompanying drawings illustrate a principle of the aforesaid method, proposed by the assignee of the present application, in which the edge position is stepwise shifted and information is recorded. Initially, a PWM (pulse width modulation) signal (see FIG. 1B) is generated in response to recording data and a pit (see FIG. 1A) corresponding to the length provided when the recording signal is at its zero-cross point is formed. With this arrangement, the edge position of the pit is changed stepwise from a position shown by a reference clock (see FIG. 1C). In association with the amount of such change, data of eight stages (3 bits) from 0 to 7 can be recorded per edge.
FIGS. 2A through 2D are diagrams used to explain the principle by which the signal thus recorded is reproduced. Initially, an RF signal (see FIG. 2A) reproduced from an information recording medium is amplified to obtain a binary RF signal (see FIG. 2B). A clock pit is formed on the disc in which information has been recorded and a reference clock (see FIG. 2C) is generated on the basis of the clock pit. A sawtooth wave signal (see FIG. 2D) is generated in synchronism with the reference clock. Then, the edge position of the information pit is detected by detecting the timing at which the sawtooth wave signal and the binary RF signal cross each other.
Magneto-optical recording systems use an MO disc in which a pit is formed by a partial inversion of magnetization on a magneto-optical film. When the recording method in which the edge position of the information pit is shifted stepwise from the predetermined position in response to the recording information is applied to a magneto-optical recording system, the following disadvantages must be overcome.
Magneto-optical disc recording systems include magnetic field modulation system and a light modulation system. While a magnetic field modulation system is capable of overwrite, the magnetic field modulation system must modulate the magnetic field. There is then the drawback that the magnetic field modulation system is large in energy loss. Also, the magnetic field modulation system needs much energy because a laser is continuously energized.
On the other hand, while the light modulation system is small in energy loss because a laser is energized intermittently, the light modulation system is incapable of overwrite. In order to effect the overwrite, some recording media are formed as a multi-layer structure. According to this system, two magnets are required to record data on such recording media and laser powers of three levels are required.
The conventional magneto-optical disc recording method that is capable of the overwrite is large in energy loss and makes the recording apparatus become complicated in arrangement.